I can't think of a JIS screw that would be on your exhaust. Can you show a picture of this?
Edit: Are you talking about #10 on this diagram? https://www.partzilla.com/catalog/honda ... 50/muffler
If so, that only holds on the heat shield.
Am I understanding correctly that you removed the bolts holding the muffler to the frame by the rear foot pegs? If so, are they loose at least? Sometimes the exhaust collars can be rusted in. If you spray them with PB Blaster, wait a few minutes, then raise and lower the pipes at the muffler end. Working them up and down like that usually breaks them loose.
Note that if you remove the exhaust, you will need to replace the copper exhaust gaskets. They can not be reused, and attempting to do so will cause exhaust leaks and back-firing. Once you get the pipes off, you may not even realize there were gaskets there. They will usually be flattened out and stuck on the cylinder head where the pipes came off. A small screwdriver will pry out the gasket fairly easily.